1. Field of the Invention
The present invention relates to a fuel evaporative gas emission suppression system, and particularly to a technique for detecting an abnormality of the fuel evaporative gas emission system.
2. Description of the Related Art
Conventionally, to prevent emission of fuel evaporative gas evaporated in a fuel tank into the atmosphere, there is provided a fuel evaporative gas emission suppression system comprising: a canister mounted in a communication path for communicating a fuel tank with an intake passage of an internal combustion engine; a changeover valve for releasing or blocking the canister to or from the atmosphere; a sealing valve for communicating or blocking between the fuel tank and the canister; and a purge valve for communicating and shutting off the communication path between the intake passage and the canister. The fuel evaporative gas emission suppression system opens the changeover valve and the sealing valve and closes the purge valve during fueling so as to make fuel evaporative gas in the fuel tank flow out into the canister, thereby causing the fuel evaporative gas to adsorb to activated carbon placed in the canister. Then, the fuel evaporative gas emission suppression system performs purge treatment, whereby the changeover valve and the purge valve are opened while the internal combustion engine is in operation so that the fuel evaporative gas adsorbed to the activated carbon in the canister is discharged to the intake passage of the internal combustion engine, thereby treating the fuel evaporative gas.
Further, there have been developed techniques for detecting a leakage of fuel evaporative gas from a fuel evaporative gas emission suppression system, and a failure of a valve in the same system.
For example, there is a technique which is configured to produce negative pressure in a purge passage and a fuel tank by controlling opening/closing of a changeover valve, a sealing valve and a purge valve when the internal combustion engine is in operation and by taking advantage of negative pressure generated in the intake passage of the internal combustion engine, and to detect an abnormality such as a leakage and a failure of a valve based on whether the negative pressure is retained or not retained.
However, in a vehicle such as a plug-in hybrid vehicle which is provided with an electric motor in addition to an internal combustion engine, and which travels mainly by the driving force of the electric motor, there is less chance that the internal combustion engine is operated, to improve fuel economy, and therefore there will be less chance that abnormality detection can be performed when performing abnormality detection of a fuel evaporative gas emission suppression system while the internal combustion engine is in operation.
Accordingly, as a fuel evaporative gas emission suppression system to be provided in a vehicle in which the internal combustion engine has less chance to be operated, there is an example which includes a negative pressure pump capable of reducing the pressure in passages of the fuel evaporative gas emission suppression system, and controls the operation of the negative pressure pump and the opening/closing of a changeover valve, a sealing valve and a purge valve while the internal combustion engine is stopped, thereby performing abnormality detection of the fuel evaporative gas emission suppression system based on changes of intake pressure of the negative pressure pump and pressure in the fuel tank (see Japanese Patent No. 4352945).
Further, there is developed a fuel evaporative gas emission suppression system in which an opening/closing valve (canister opening/closing valve) is provided between the communication path and the canister instead of the canister being directly mounted in the communication path in the fuel evaporative gas emission suppression system having a negative pressure pump as shown in the above described patent publication.
In this fuel evaporative gas emission suppression system, by closing the canister opening/closing valve, it is made possible to discharge fuel evaporative gas from the fuel tank into an intake passage of the internal combustion engine via a communication path without making it flow into the canister, thus making it possible to restrict fuel evaporative gas from adsorbing to the canister.
Then, in a fuel evaporative gas emission suppression system having a canister opening/closing valve as described above, it is demanded to detect abnormalities such as open sticking and closed sticking of the canister opening/closing valve.
Moreover, in the fuel evaporative gas emission suppression system according to the above described patent publication, purge flow monitoring is enabled which determines that purge treatment is possible based on a fact that pressure in the canister decreases by not less than a predetermined value when the purge valve is released when the internal combustion engine is in operation.
In such a fuel evaporative gas emission suppression system in which purge flow monitoring is possible and moreover a canister opening/closing valve is provided, it is demanded to perform not only purge flow monitoring, but also detection of abnormalities such as open sticking of the canister opening/closing valve.